1. Field
Aspects of the present invention generally relate to a sheet processing apparatus and an image forming apparatus having a binding processing function.
2. Description of the Related Art
A stapling device has conventionally been used widely as a device for binding sheets on which images are formed by an image forming apparatus such as a copying machine and a printer. The stapling device performs binding processing to bind a sheet bundle including a plurality of sheets by using a binding member such as metal staples. However, when using each sheet of the sheet bundle stapled by the stapling device as a document to be read, the staples binding the sheet bundle need to be removed. When recycling the sheet bundle bound by staples, the staples binding the sheet bundle also need to be removed to separately collect the sheets and the staples from the viewpoint of environmental protection. Since the staples used for the binding processing are discarded after being used, there has been a problem in terms of reuse of resources.
Japanese Patent Application Laid-Open No. 2004-155537 discusses a sheet binding device that uses no binding member such as a staple to reduce time and effort when reusing the sheets as a document or at the time of recycling. Using no staples, such a sheet binding device discards no staples. The sheet binding device is configured to, after a plurality of sheets conveyed from an image forming apparatus is bundled and aligned into a sheet bundle, press against the sheets a tooth die having protrusions and recesses for forming recesses and protrusions in part of the sheet bundle. The sheet binding device performs binding processing by thus pressing the sheet bundle to entangle fibers of the sheet bundle with each other.
If the conventional stapleless binding method described above is applied to an image forming apparatus, an actuator can be used as a driving source for pressing the tooth die having protrusions and recesses against the sheet bundle to automate the pressing operation. In the stapleless binding processing, steady application of constant pressing force to the sheet bundle is important in maintaining the quality of the sheet bundle after undergoing the binding processing so that the retention force of the binding portion lasts and the bound portion will not get broken. However, actuators have individual variations in output torque characteristics even under a constant operating condition (constant driving voltage and driving current). This leads to variations in the pressing force applied to the sheet bundle. Aside from individual variations, the output torque characteristics of the actuator also vary with temperature of an operating environment, use time, and use frequency of the image forming apparatus. Consequently, if the output torque is low, a phenomenon in which the sheet bundle easily exfoliates (hereinafter, referred to as poor binding) can occur. If the output torque is high, the application of excessive pressure to the sheet bundle can break the sheets. In addition, since high output torque produces pressure more than needed for the binding processing, not only the tooth die but also the mechanism linked to the tooth die needs to have high strength. Therefore, cost for improving the strength of the materials and a size of the mechanism are increased.